There are many cases where it is necessary to pick off a pressure, particularly of a fluid, and to utilize it either for measuring or for generating encodable signals.
For example, sound is produced the grooves of a record, which react to the variable pressure, applied to the grooves a reading head; atmospheric pressure is measured by of barometers and altimeters weight is measured by; the pressure of a body placed on a scale; the blood pressure in the arteries of a living body is measured for gaining medical information etc.
Among the numerous cases of pressure detection there if one which is particularly important and that is detection of anterial pressure. As a matter of fact, when an operation on the human body is to be carried out, not only physiological data are taken into account but also psychological factors concerning especially the possibility of pain which such measuring can cause.
Up to now the most common means for blood pressure detecting has been the one in which the artery to be examined is cut off completely with the help of an inflatable sleeve or garrote. It is clear that such a means only allows the measuring of blood pressure in some parts of the body.
Other means of picking off pressure have therefore been devised, like the one described in U.S. Pat. No. 3,704,708.
However, this disclosure is different from the present invention.
As a matter of fact, U.S. Pat. No. 3,704,708 is based on the principle that the device does not depend on the condition of the vascular system but only on the true transmission of a hydrostatic pressure (col. 4, lines 33 to 37 and col. 7, lines 5 to 10).
The functioning of the device requires that the examined artery be flattened (col. 4, lines 40 and 41) and needs the presence of a skeletal bone (col. 3, lines 44 and 45).
For this reason the capillary tube 110 is provided with a movable stopper 125 (col 6, line 48), by means of which it can be temporarily closed (col. 6, lines 66 and 67).
The stopper 124 is replaced after the liquid 126 (which is subjected to atmospheric pressure) rises, so that there is no counter pressure at all. If the stopper 124 is replaced before rising of the liquid, the counter pressure from the captured air has an unknown value which varies upon each measurement since the volume of air is variable (the liquid 126 rises more or less when the stopper 124 is replaced).